Flexible method for the joint production of (i) formic acid, (ii) a carboxylic acid comprising at least two carbon atoms and/or the derivatives thereof, and (iii) a carboxylic acid anhydride

ABSTRACT

Disclosed is a method for jointly producing (i) formic acid (III); (ii) a carboxylic acid comprising at least two carbon atoms (II) and/or the derivatives thereof; and (iii) a carboxylic acid anhydride (VII). According to said method, (a) a formic acid ester (I) is transesterfied to formic acid (III) and the corresponding carboxylic acid ester (IV) by means of a carboxylic acid comprising at least two carbon atoms (II); (b) at least one portion of the carboxylic acid ester (IV) formed in step (a) is carbonylated in order to obtain the carboxylic acid anhydride (V); and (c) at least one portion of the carboxylic acid anhydride (V) formed in step (b) is further dehydrated by means of a carboxylic acid (VI) so as to form a carboxylic acid anhydride (VII) and the carboxylic acid (II).

Flexible process for the joint preparation of (i) formic acid, (ii) acarboxylic acid having at least two carbon atoms and/or derivativesthereof and (iii) a carboxylic anhydride.

The present invention relates to a process for the joint preparation of(i) formic acid, (ii) a carboxylic acid having at least two carbon atomsand/or derivatives thereof, for example a carboxylic ester or acarboxylic anhydride, and (iii) a further carboxylic anhydride.

Formic acid is an important compound which has a wide variety of uses.It is used, for example, for acidification in the production of animalfeed, as preservatives, as disinfectant, as auxiliary in the textile andleather industry and as synthetic building block in the chemicalindustry.

The most important processes for preparing formic acid are indicatedbelow (cf. Ullmann's Encyclopedia of Industrial Chemistry, 6^(th)edition, 2000 electronic release, Chapter “FORMIC ACID—Production”).

The industrially most important process for preparing formic acid ishydrolysis of methyl formate and subsequent concentration of the aqueousformic acid solution obtained. Known processes which may be mentionedare the Kemira-Leonard process and the BASF process. A greatdisadvantage of these processes is the formation of an aqueous formicacid solution as a result of the hydrolysis step, which results in aseries of further disadvantages. Thus, complicated concentration of theformic acid solution by extractive rectification using an entrainer isrequired. As a result of the presence of water, the aqueous orconcentrated formic acid solution to be handled is highly corrosive andrequires the use of expensive materials of construction for the plantcomponents concerned. The processes mentioned therefore suffer from highcapital and operating costs, from a technically complicated andextensive construction of the production plant, a high energyconsumption and a not inconsiderable residual water content in theconcentrated formic acid.

The oxidation of hydrocarbons, for example butanes or naphtha, forms abroad range of products which includes formic acid and has to beseparated and concentrated in a complicated manner. This process, too,suffers from the disadvantage of an extractive rectification of thecrude formic acid using an entrainer being necessary. The abovementioneddisadvantages resulting from the water content of are also present.

In an older process, formic acid is obtained by hydrolysis of formamide,which can be obtained by ammonolysis of methyl formate by means ofammonia. Hydrolysis is carried out using sulfuric acid and water.Disadvantages of this process are the undesirable formation of ammoniumsulfate as coproduct and the presence of water, which leads to theabovementioned disadvantages.

Carboxylic acids such as acetic acid and its higher homologues and thecorresponding anhydrides are important and versatile compounds. They areused, for example, for the preparation of esters, carboxylic anhydrides,as additives in the polymer sector or as intermediates in thepreparation of textile chemicals, dyes, plastics, agrochemicals andpharmaceuticals. The low molcular weight homologues acetic acid andpropionic acid are particularly important.

The most important processes for preparing acetic acid and its higherhomologues are indicated below (cf. Ullmann's Encyclopedia of IndustrialChemistry, 6^(th) edition, 2000 electronic release, Chapter “ACETICACID—Production” and Chapter “CARBOXYLIC ACIDS, ALIPHATIC—Production”).

The industrially most important process for preparing acetic acid iscarbonylation of methanol in the presence of suitable carbonylationcatalysts, for example cobalt carbonyl, iridium carbonyl or rhodiumcarbonyl compounds. Known processes which may be mentioned are the BASFprocess and the Monsanto process. A disadvantage of these processes isthe presence of water in the reaction medium, which as a result of thewater gas shift reaction of water and carbon monoxide to form carbondioxide and hydrogen reduces the yield derived from the carbon monoxideused. Furthermore, a high energy input is necessary in the work-up bydistillation because of the water content. In addition, the processesmentioned suffer from high capital and. operating costs and require atechnically complicated and extensive construction of the productionplant.

The oxidation of hydrocarbons, for example ethane, butanes or naphtha,forms a broad range of products which comprise acetic acid and possiblyhigher homologues and have to be separated and concentrated in acomplicated manner. The abovementioned disadvantages resulting from thewater content also apply.

The synthesis of carboxylic acids by oxidation of the correspondingaldehydes starts out-from expensive olefin as feedstock. Thus,acetaldehyde is prepared industrially by oxidation of ethene by theWacker process and its higher homologues are obtained byhydroformylation of ethene, propene, etc. These processes therefore havean economically unattractive raw materials basis.

Carboxylic esters, in particular methyl acetate, are important solvents.Methyl acetate is used, for example, for dissolving nitrocellulose oracetylcellulose. Vinyl acetate is used widely in the preparation ofpolymers and copolymers.

There is a great variety of processes for preparing carboxylic esters(cf. Ullmann's Encyclopedia of Industrial Chemistry, 6^(th) edition,2000 electronic release, Chapter “ESTERS, ORGANIC—Production”). Mentionmay be made of the esterification of carboxylic acids with alcohols, thereaction of carboxylic chlorides or carboxylic anhydrides with alcohols,the transesterification of carboxylic esters, the reaction of keteneswith alcohols, the carbonylation of olefins by means of carbon monoxideand alcohols, the condensation of aldehydes, the alcoholysis of nitrilesand the oxidative acylation of olefins.

Alkyl acetates are obtained mainly by esterification of acetic acid oracetic anhydride with alkanols. Methyl acetate is also formed asby-product in the synthesis of acetic acid (cf. Ullmann's Encyclopediaof Industrial Chemistry, 6^(th) edition, 2000 electronic release,Chapter “ACETIC ACID—Production”). A further possible way ofsynthesizing methyl acetate is the carbonylation of dimethyl ether (cf.Ullmann's Encyclopedia of Industrial Chemistry, 6^(th) edition, 2000electronic release, Chapter “ACETIC ANHYDRIDE AND MIXED FATTY ACIDANHYDRIDES—Acetic Anhydride—Production”). A disadvantage of the latterprocess is the use of expensive dimethyl ether.

Acetic anhydride is an important synthetic building block in thechemical industry and is used, for example, for preparing acetylcelluloses, acetylsalicylic acid, acetanilide, sulfonamides or vitaminB6.

The most important processes for preparing acetic anhydride areindicated below (cf. Ullmann's Encyclopedia of Industrial Chemistry,6^(th) edition, 2000 electronic release, Chapter “ACETIC ANHYDRIDE ANDMIXED FATTY ACID ANHYDRIDES—Acetic Anhydride—Production”).

One industrially important process for preparing acetic anhydride is thereaction of acetic acid with ketene obtained in a previous step bythermal elimination of water from acetic acid. Disadvantages of thisprocess are the very high energy consumption caused by the thermalpreparation of ketene and the need to handle the extremely toxic ketene.

In a further industrially important process for preparing aceticanhydride, methanol is converted into methyl acetate by carbonylationand esterification in a first step and this is carbonylated in a secondstep to produce acetic anhydride.

A further process for preparing acetic anhydride is the liquid-phaseoxidation of acetaldehyde. A disadvantage of this process is the use ofexpensive acetaldehyde which is obtained industrially by oxidation ofethene in the Wacker process. This process therefore has an economicallyunattractive raw materials basis.

A further process for preparing acetic anhydride is the carbonylation ofmethyl acetate in the presence of a transition metal catalyst. Methylacetate is generally obtained as by-product in the synthesis of aceticacid and by esterification of acetic acid with methanol.

EP-A 0 087 870 teaches an integrated process for preparing aceticanhydride and acetic acid from methanol and carbon monoxide. In a firststep, acetic acid is esterified with methanol to form methyl acetatewhich is carbonylated in the presence of water in a second step to givea mixture comprising acetic anhydride and acetic acid. The mixtureobtained is worked up by distillation, with the required amount ofacetic acid being fed to the first stage. The remaining amount of aceticacid and acetic anhydride is taken off as product. Disadvantages of thisprocess are the formation of stoichiometric amounts of water in theesterification step and the associated problems occurring when handlingwater-containing acetic acid and in its work-up. The abovementioneddisadvantages resulting from the water content apply.

Carboxylic anhydrides are important starting materials for other acidderivatives and are also used as solvents and as dehydrating agents.Anhydrides of unsaturated aliphatic carboxylic acids, in particularacrylic acid and methacrylic acid, are also important starting compoundsfor the preparation of interesting monomers which are:difficult toobtain by other synthetic routes. Aromatic carboxylic anhydrides, forexample 1,2,4,5-benzenetetracarboxylic dianhydride (pyromelliticanhydride) or 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, areimportant starting materials for the preparation of heat-resistantresins, for example polyamide or epoxy resins.

Various methods of preparing carboxylic anhydrides are known. Anoverview of the three principal synthetic routes may be found, forexample, under the keyword “Sätureanhydride” in CD Römpp Chemie Lexikon,Version 1.0, Stuttgart/New York, Georg Thieme Verlag 1995. In the firstsynthetic route, the parent carboxylic acids are used and water iseliminated by use of water-withdrawing substances, for example P₄O₁₀, orby heating, thus forming the carboxylic anhydride. Disadvantages of thissynthetic route are the use of starting materials whose preparationrequires a great deal of energy (e.g. P₄O₅₁₀) and the formation ofundesirable coproducts (e.g. phosphoric acid when P₄O₁₀ is used). Adisadvantage of the thermal elimination of water is the risk offormation of undesirable by-products as a result of thermaldecomposition. In the second synthetic route, acid chlorides, forexample acetyl chloride or benzoyl chloride, are reacted with the alkalimetal salts of the corresponding carboxylic acids. A process of thistype is described, for example, in WO 95/32940. Disadvantages of thissynthetic route are the use of an acid chloride which is a startingmaterial whose preparation requires a great deal of energy and theformation of alkali metal chloride and the alkali metal salt of the acidchloride used as undesirable coproducts. In the third synthetic route,the parent carboxylic acids are transanhydrided with acetic anhydride orketene. Details of this synthetic route are given, for example, in DE-A35 10 035, EP-A 0 231 689, DE-A 36 44 222 and EP-A 1 231 201. Adisadvantage of this synthetic route is the use of acetic anhydride orketene which firstly have to be obtained by the energy-intensive processdescribed above for acetic anhydride and ketene.

It is an object of the present invention to find a process for preparingcarboxylic acids and/or derivatives thereof which no longer has theabovementioned disadvantages, has a readily available and economicallyattractive raw materials basis, makes a simple and inexpensiveconstruction of the plant possible (low capital costs), avoidsundesirable by-products as a result of coproduction and has a low energyconsumption and favorable operating costs. A further object is to find aprocess which also makes it possible to prepare anhydrous carboxylicacids if required and thus makes the handling of less corrosive mediaand the use of less expensive materials of construction possible and asa result of the lower corrosivity also offers a higher degree of safety.Another object is to find a process which, quite generally, makes itpossible to prepare a wide variety of carboxylic anhydrides, inparticular unsaturated carboxylic anhydrides such as acrylic anhydrideor methacrylic anhydride.

We have found that this object is achieved by a process for the jointpreparation of

(i) formic acid (III);

(ii) a carboxylic acid having at least two carbon atoms (II) and/orderivatives thereof; and

(iii) a carboxylic anhydride (VII), which comprises

(a) transesterifying a formic ester (I) with a carboxylic acid having atleast two carbon atoms (II) to form formic acid (III) and thecorresponding carboxylic ester (IV); (b) carbonylating at least part ofthe carboxylic ester (IV) formed in step (a) to form the correspondingcarboxylic anhydride (V); and

(c) transanhydriding at least part of the carboxylic anhydride (V)formed in step (b) with a carboxylic acid (VI) to form a carboxylicanhydride (VII) and the carboxylic acid (II).

In step (a), a formic ester (I) is reacted with a carboxylic acid havingat least two carbon atoms (II) to form formic acid (III) and thecorresponding carboxylic ester (IV).

The formic ester used has the formula (I)

where the radical R¹ is an organic radical. The organic radical ispreferably an unsubstituted or substituted, aliphatic, aromatic oraraliphatic radical which has from 1 to 12 carbon atoms and may containone or more heteroatoms such as oxygen, nitrogen or sulfur, for example—O—, —S—, —NR—, —CO— and/or —N=in aliphatic or aromatic systems, and/orbe substituted by one or more functional groups which may contain, forexample, oxygen, nitrogen, sulfur and/or halogen, for example byfluorine, chlorine, bromine, iodine and/or a cyano group.

Formic esters are generally obtainable by base-catalyzed carbonylationof the corresponding alcohols and by esterification of the correspondingalcohols with formic acid (cf. Ullmann's Encyclopedia of IndustrialChemistry, 6^(th) edition, 2000 electronic release, Chapter “FORMICACID—Derivatives”). The simplest representative of this class ofcompounds, methyl formate, is obtained industrially by carbonylation ofmethanol.

For the purposes of the present invention, a carboxylic acid having atleast two carbon atoms (II) is a carboxylic acid which bears a radicalhaving at least one carbon atom on the carboxyl group. The carboxylicacids used have the formula (II)

where the radical R²is an organic radical. The preferred organic radicalR² is as defined in the case of R¹.

The abovementioned transesterification reaction in step (a) is anequilibrium reaction which is generally catalyzed by the presence of acatalyst.

In the process of the present invention, step (a) can be carried outusing known methods of transesterification (cf. Ullmann's Encyclopediaof Industrial Chemistry, 6^(th) edition, 2000 electronic release,Chapter “ESTERS, ORGANIC—Chemical Properties” and “ESTERS,ORGANIC—Production” and the references below).

In general, small amounts of acidic or basic substances are used ascatalyst. Preference is given to using acids and acidic solids. Exampleswhich may be mentioned are strong protic acids, for example sulfuricacid, perchloric acid, benzenesulfonic acid, p-toluene sulfonic acid,molybdophosphoric acid and tungstosalicic acid; acidic ion exchangers,for example ion exchangers containing perfluorinated sulfonic acidgroups (SU-A 1,432,048); and also acidic oxides, for example zeolites(DE-A 35 06 632), aluminosilicates (US 3,328,439) or SiO₂/TiO₂ (DE 27 10630). Preferred catalysts are mineral acids, p-toluenesulfonic acid andzeolites.

If strong protic acids are used as homogeneous catalysts, theirconcentration in the reaction mixture is generally from 0.01 to 50% byweight, preferably from 0.01 to 2% by weight.

As cocatalyst to be used together with the abovementioned catalysts, itis possible to use water or methanol, generally in an amount of up to20% by weight, based on the reaction solution. However, it should benoted that an increase in the water content also increases thecorrosivity of the reaction medium and makes the work-up of the productsmore difficult. It may therefore be advantageous to carry out thetransesterification without addition of water or methanol as cocatalyst.If the transesterification is carried out in the presence of water ormethanol, it may be advantageous to add carboxylic anhydride (V) to thereaction product mixture in order to bind the water. This can, forexample, be added directly at the reactor outlet or in the column (e.g.bottom of the column). This measure also makes it possible to prepareanhydrous formic acid and an anhydrous carboxylic ester (IV) in atransesterification cocatalyzed by water or methanol. Anhydrous formicacid and anhydrous carboxylic ester (IV) can also be prepared withoutproblems in this way when using methanol-containing methyl formate asformic ester (I). The typical residual methanol content of about 2-4% byweight when using methyl formate as formic ester (I) is found to beadvantageous owing to its property as cocatalyst.

The transesterification can be carried out either in the liquid phase orin the gas phase. In the case of a transesterification in the gas phase,preference is given to using heterogeneous catalysts such as theabovementioned ion exchangers or acidic oxides. In a transesterificationin the liquid phase, homogeneous or heterogeneous catalysts are used.The transesterification is preferably carried out in the liquid phase.

In general, the transesterification is carried out at from 20 to 300°C., preferably from 50 to 180° C.

The pressure is generally from 0.1 to 5 MPa abs.

The transesterification can be carried out in the presence of anadditional inert, polar solvent. For the purposes of the presentinvention, inert solvents are solvents which do not react chemicallywith the starting materials, the products or the catalysts under thereaction conditions employed. Examples of suitable solvents arepolyethers. Solvents are generally used in transesterifications in whichstarting materials and/or products which are insufficiently soluble inthe solvent-free reaction mixture at the desired temperature, thedesired pressure and the desired ratios of starting materials andproducts are present. If the starting materials and the products arealso soluble in the solvent-free reaction mixture under the selectedconditions, the transesterification is preferably carried out withoutaddition of a solvent.

The starting materials formic ester (I) and carboxylic acid (II) aregenerally each added in the stoichiometric amount.

Additional addition of one of the two starting materials, for example asan initial charge prior to the commencement of the reaction, enables anonstoichiometric ratio of the two starting materials to be set in atargeted manner in the reaction mixture. In this way, for example, astarting material which has good solvent properties can improve thesolubility of the other starting material or the product. It is likewisepossible to maintain an appropriate excess of one of the two products inthe reaction mixture.

The transesterification can be carried out batchwise or continuously.Preference is given to a continuous process.

In the process of the present invention, it is in principle possible tocarry out the transesterification in any reaction apparatus known fortransesterification reactions. Suitable reaction apparatuses for areaction in the liquid phase are, for example, stirred tank reactors,distillation columns, reactive columns and membrane reactors. To achievea high conversion, it is advantageous for at least one of the twoproducts, preferably both, to be removed continually from the reactionmixture. When a stirred tank reactor is used, this is achieved, forexample, by continuously taking off reaction mixture, subsequentlyseparating the two products and recirculating the two unreacted startingmaterials and, if appropriate, the catalyst. When a distillation columnis used, the transesterification reaction occurs in the liquid phase,with the lower-boiling components being able to be separated off bydistillation and, depending on whether they are starting materials orproduct, recirculated or discharged. When a reactive column is used, thepreferably heterogeneous catalyst is located in the separation region ofthe column. In a manner similar to the case of the distillation columndescribed, the lower-boiling components are in this case separated offby distillation and recirculated or discharged.

Examples of suitable reaction apparatuses for a reaction in the gasphase are flow tubes or shaft reactors.

The separation of the reaction mixture can be carried out in variousways. The method is generally determined by the properties of thestarting materials and products to be separated. Examples of possibleseparation methods are distillation, crystallization and extraction. Itmay be pointed out that combinations of various separation methods arealso possible, including when a distillation column or reactive columnhas previously been used for the transesterification. In general,preference is given to separation by distillation, which can also becarried out under reduced pressure or in vacuo. If separation bydistillation is not possible or possible only with great difficulty, forexample in the case of relatively high-boiling or readily decomposablecomponents, the alternative processes mentioned become important. Asuitable work-up concept can be readily developed by a person skilled inthe art from a knowledge of the starting materials, products andpossibly the catalyst present.

Owing to its good distillation properties, formic acid (III) ispreferably removed by distillation.

The preferred separation by distillation of the reaction mixtureobtained is generally carried out, using three distillation columns ortheir equivalents (e.g. a dividing wall column and a distillationcolumn) to obtain a separation into four streams. The stream comprisingthe formic ester (I) is generally recirculated to thetransesterification, the stream comprising the carboxylic ester (IV) ispartly or entirely passed to the carbonylation step (b), the formic acid(III) is discharged from the system as product and the remaining streamcomprising the carboxylic acid (II) is generally likewise recirculatedto the transesterification.

Since any formic ester (I) still present is isomerized to thecorresponding carboxylic acid R¹—COOH in the presence of thecarbonylation catalyst in the subsequent carbonylation of the carboxylicester (IV) to the carboxylic anhydride (V), it may be possible, in avariant with a simplified work-up by distillation saving a distillationcolumn, to take off not only a stream comprising the formic ester (I), astream comprising formic acid (III) and a stream comprising thecarboxylic acid (II) but also a further stream comprising the formicester (I) and the carboxylic ester (IV) and recirculate this to thecarbonylation step (b). This latter stream can, for example, be obtainedat a side offtake of the first distillation column.

In the process of the present invention, the total amount of thecarboxylic ester (IV) obtained or only part thereof can be fed to thecarbonylation step (b). In the latter variant, part of the carboxylicester (IV) formed can be obtained as end product. The remaining part ofthe carboxylic ester (IV) is passed to the carbonylation step (b).

In step (b), at least part, preferably at least 5%, particularlypreferably at least 10% and very particularly preferably at least 50%,of the carboxylic ester (IV) formed in step (a) is carbonylated in thepresence of a catalyst to give the corresponding carboxylic anhydride(V).

In the process of the present invention, step (b) can be carried outusing known methods of carbonylating carboxylic esters (cf. Ullmann'sEncyclopedia of Industrial Chemistry, 6^(th) edition, 2000 electronicrelease, Chapter “ACETIC ANHYDRIDE AND MIXED FATTY ACIDANHYDRIDES—Acetic Anhydride—Production” and the references below).

As catalysts, it is generally possible to use metals of groups 8 to 10of the Periodic Table and their compounds in the presence of halides andorganic halogen compounds. Preferred catalyst metals are rhodium,iridium, palladium, nickel and cobalt, in particular rhodium (EP-A 0 677505). As halides or organic halogen compounds, use is generally made ofiodine compounds. Preference is given to adding alkali metal, iodidesand alkaline earth metal iodides (U.S. Pat. No. 5,003,104, U.S. Pat. No.4,559,183), hydrogen iodide, iodine, iodoalkanes, in particulariodomethane (methyl iodide) (GB-A 2,333,773, DE-A 24 41 502), orsubstituted azolium iodide (EP-A 0 479 463). The catalyst metals aregenerally stabilized by ligands. As ligands, preference is given tousing nitrogen and phosphorus compounds such as N-containingheterocyclic compounds (DE-A 28 36 084), amines, amides (DE-A 28 44 371)or phosphines (U.S. Pat. No. 5,003,104, EP-A 0 336 216). The catalystsystems can further comprise promoter metals, for example chromium inthe nickel/chromium system (U.S. Pat. No. 4,002,678), ruthenium in theiridium/ruthenium system (GB-A 2,333,773) or cobalt in theruthenium/cobalt system (U.S. Pat. No. 4,519,956). Preferred catalystsystems are systems comprising rhodium and/or iridium, methyl iodide,nitrogen and/or phosphorus-containing ligands and, if desired,promoters, for example lithium or chromium. Particular preference isgiven to using a catalyst based on rhodium triiodide, lithium iodide andiodomethane, for example as described in U.S. Pat. No. 4,374,070.

The catalyst can be used in unsupported form as homogeneous catalyst orin supported form as heterogeneous catalyst. Suitable support materialsare, for example, inorganic oxides such as silicon dioixide or aluminumoxide (EP-A 0 336 216), or polymers such as ion exchangers (J6 2135 445)or resins (JP 09 124 544).

The carbonylation can be carried out in the presence of hydrogen (U.S.Pat. No. 5,003,104, GB-A 2 333 773, U.S. Pat. No. 4,333,885, WO82/01704) or in the absence of hydrogen (A. C. Marr et al., Inorg. Chem.Comm. 3, 2000, pages 617 to 619). It is generally advantageous to carryout the carbonylation in the presence of hydrogen, generally usinghydrogen concentrations from the ppm range up to 15% by volume,preferably from 1 to 10% by volume, based on the gaseous feed streamemployed.

The carbonylation can be carried out either in the gas phase (EP-A 0 336216) or in the liquid phase. When it is carried out in the gas phase,use is generally made of supported catalysts. In the process of thepresent invention, preference is given to the carbonylation beingcarried out in the liquid phase.

The carbonylation in the gas phase is generally carried out at from 130to 400° C., preferably from 150 to 280° C., and a pressure of from 0.1to 15.MPa abs, preferably from 0.5 to 3 MPa abs. The carbonylation inthe liquid phase is generally carried out at from 100 to 300° C.,preferably from 170 to 200° C., and a pressure of from 0.1 to 15 MPaabs, preferably from 1 to 8 MPa abs.

When the carbonylation is, as preferred, carried out in the liquid phaseand a homogeneous catalyst is used, the catalyst concentration employedis generally in the range from 0.01 to 1% by weight, based on thereaction solution.

The carbonylation can be carried out in the presence of an additionalinert solvent. For the purposes of the present invention, inert solventsare solvents which do not react chemically with the starting compounds,the products or the catalysts under the reaction conditions employed.Suitable inert solvents are, for example, aromatic and aliphatichydrocarbons and also carboxylic acids and their esters. Preference isgiven to using solvents in carbonylations in which the starting materialand/or the product is insufficiently soluble in the solvent-freereaction mixture at the desired temperature and/or the desired pressure.If the starting materials and the products are also soluble in thesolvent-free reaction mixture under the selected conditions, thetransesterification is preferably carried out without addition of asolvent.

The carbonylation can be carried out batchwise or continuously.Preference is given to a continuous process.

In the process of the present invention, the carbonylation can inprinciple be carried out using any reaction apparatuses known forcarbonylation reactions. The carbonylation in the gas phase is generallycarried out in a flow tube or shaft reactor. Suitable reactionapparatuses for the

preferred carbonylation in the liquid phase are, for example, stirredtank reactors, jet loop reactors and bubble columns. Their use in acontinuous process is briefly described below.

When the abovementioned reaction apparatuses are used, the desiredamounts of carboxylic ester (IV) and carbon monoxide are generallypassed continuously into the reaction solution comprising, inparticular, the carboxylic anhydride (V), the carbonylation catalystand, if desired, an additional solvent with intensive mixing. The heatof carbonylation evolved can, for example, be removed by means ofinternal heat exchangers, by cooling the wall of the reaction apparatusand/or by continuously taking off the hot reaction solution, cooling itexternally and recirculating it. When a jet loop reactor or a bubblecolumn is used, an external circuit is necessary to ensure mixing. Theproduct is taken off by continuously taking off reaction mixture andsubsequently separating off the carbonylation catalyst in a suitableseparation apparatus. A suitable separation apparatus is, for example, aflash evaporator in which the carboxylic anhydride (V) is vaporized bypressure reduction. The remaining solution, which comprises thecarbonylation catalyst, is returned to the reaction apparatus. Undersuitable temperature and pressure conditions, it may also be possiblefor the carboxylic anhydride formed to be taken off continuously fromthe reaction solution by vaporization (DE-A 30 24 353). The vaporizedcarboxylic anhydride (V) can, depending on requirements, be passed to awork-up step or a subsequent step for further reaction. In the case ofrelatively high-boiling carboxylic anhydrides (V) for which the flashevaporation described is not possible because of their low volatility,the reaction product mixture is worked up in other ways, for example bydistillation under reduced pressure, by crystallization or byextraction.

The process parameters and measures to be chosen in the process of thepresent invention are dependent, inter alia, on the nature of thecarboxylic ester (IV) used, the carboxylic anhydride (V) formed and thecatalyst system selected and can be determined using customary technicalskills.

Depending on the formic ester (I) and carboxylic acid (II) chosen asstarting materials, the carbonylation in step (b) forms a symmetrical orunsymmetrical carboxylic anhydride, i.e. the radicals R¹ and R² can beidentical or different.

Furthermore, it is possible to add an alcohol R¹—OH or R²—OH to thecarboxylic ester (IV) to be carbonylated. The alcohol is then convertedinto the corresponding carboxylic acid R¹—COOH or R²—COOH (II). Such anaddition makes it possible to increase the ratio of the carbonylationproducts R²—COOH (II), carboxylic anhydride (V) and R¹—COOH to formicacid (I). Thus, for example, the additional introduction of methanol inthe carbonylation of methyl acetate leads to formation of acetic acid inaddition to acetic anhydride from the carbonylation of the methylacetate. It is also possible to add water, carboxylic ester (IV), formicester (I) or ethers of the formula R¹—O—R¹, R¹—O—R² or R²—O—R² asfurther components to the carboxylic ester (IV) to be carbonylated.

In the process of the present invention, the entire amount of thecarboxylic anhydride (V) obtained or only part thereof can be passed tothe transanhydridation step (c). In the latter variant, part of thecarboxylic anhydride (V) formed can be obtained as end product. Theremaining part of the carboxylic anhydride (V) is passed to thetransanhydridation step (c).

In step (c), at least part, preferably at least 5%, particularlypreferably at least 10% and very particularly preferably at least 50%,of the carboxylic anhydride (V) formed in step (b) is transanhydrided byreaction with a carboxylic acid (VI).

The carboxylic acid to be used has the formula (VI)

where the radical R³ is an organic radical. The organic radical ispreferably an unsubstituted or substituted, aliphatic, aromatic oraraliphatic radical which has from 1 to 12 carbon atoms and may containone or more heteroatoms such as oxygen, nitrogen or sulfur, for example—O—, —S—, —NR—, —CO— and/or —N=in aliphatic or aromatic systems, and/orbe substituted by one or more functional groups which may contain, forexample, oxygen, nitrogen, sulfur and/or halogen, for example byfluorine, chlorine, bromine, iodine and/or a cyano group.

The abovementioned transanhydridation in step (c) is an equilibriumreaction. The starting materials carboxylic anhydride (V) and carboxylicacid (VI) are reacted according to the following reaction scheme to givethe products carboxylic acid (II), carboxylic acid (IIa) and carboxylicanhydride (VII).

In the process of the present invention, step (c) can be carried outusing known methods of transanhydridation. Suitable methods aredescribed, for example, in DE-A 35 10 035, EP-A O 231 689, DE-A 36 44222 and EP-A 1 231 201.

To increase the reaction rate, it is generally advantageous to carry outthe transanhydridation in the presence of catalysts. Suitable catalystsare, in particular, acidic or basic substances and also suitable metalions.

If acidic substances are used as catalysts, they can in principle besolid, liquid or gaseous under the reaction conditions. Examples ofsuitable solid acidic or basic catalysts are acidic or basic ionexchangers and acidic or basic oxides,.for instance zeolites,aluminosilicates, SiO₂/TiO₂ or transition metal oxides. Suitable liquidor gaseous acidic catalysts include organic or inorganic acids whichhave a pKa which is lower than those of the carboxylic acid (VI) and thecarboxylic acid (II). As organic or inorganic acids, preference is givento using sulfuric acid, aliphatic or aromatic sulfonic acids orphosphoric acid. The amount of organic or inorganic acid isadvantageously from 0.01 to 2 mol %, preferably from 0.1 to 2 mol %,based on the carboxylic acid (VI) used.

If metal ions are used as catalysts, they are preferably metal ions ofgroups 1 to 13 of the Periodic Table. Preference is given to the ions ofcobalt, chromium, nickel, manganese, iron, lithium, sodium, potassium,magnesium, barium, calcium, copper, zinc, zirconium, titanium,lanthanum, scandium, tungsten, cerium, molybdenum, thorium, yttrium,niobium, tantalum, hafnium, rhenium, aluminum and vanadium. Theconcentration of metal ions in the reaction mixture is advantageouslyfrom 5 to 1000 ppm by weight, preferably from 50 to 500 ppm by weight.

The transanhydridation can be carried out in the liquid phase or in thegas phase. In the case of a transanhydridation in the gas phase,preference is given to using heterogeneous catalysts, for example theabovementioned ion exchangers or acidic oxides. In the case of atransanhydridation in the liquid phase, catalysts used are preferablythe abovementioned organic or inorganic acids or metal ions. Thetransanhydridation is preferably carried out in the liquid phase or inthe liquid/gas phase.

The transanhydridation is generally carried out at from 20 to 300° C.,preferably from 30 to 200° C. The pressure is generally from 0.001 to 5MPa abs, preferably from 0.01 to 0.5 MPa abs.

The transanhydridation can be carried out in the presence of anadditional inert, polar solvent. For the purposes of the presentinvention, inert solvents are solvents which do not react chemicallywith the starting materials, the products or the catalysts under thereaction conditions employed. Examples of suitable solvents are aromatichydrocarbons and polyethers. Solvents are generally used intransanhydridations in which starting materials and/or products whichare insufficiently soluble in the solvent-free reaction mixture at thedesired temperature, the desired pressure and the desired ratios ofstarting materials and products are present. If the starting materialsand products are also soluble in the solvent-free reaction mixture underthe chosen conditions, the transanhydridation is preferably carried outwithout addition of a solvent.

The starting materials carboxylic anhydride (V) and carboxylic acid (VI)are generally added in the stoichiometrically required amounts. It maybe advantageous to use an excess of carboxylic anhydride (V) to shiftthe equilibrium in the direction of the desired carboxylic anhydride(VII) and achieve a complete conversion, viewed externally, of thecarboxylic acid (VI) used. This excess of carboxylic anhydride (V) isadvantageously up to 0.5 mol per mol of carboxylic acid (VI).

The transanhydridation can be carried out batchwise or continuously.Preference is given to a continuous process with continuous introductionof the starting materials carboxylic anhydride (V) and carboxylic acid(VI) and with continuous discharge of the reaction mixture for furtherwork-up or continuous discharge of the desired product carboxylicanhydride (VII) and also the carboxylic-acids (II) and (IIa) form and,if applicable, the excess carboxylic anhydride (V).

In the process of the present invention, the transanhydridation can inprinciple can be carried out using all reaction apparatuses known fortransanhydridation reactions. Examples of suitable reaction apparatusesfor the reaction in the liquid phase are stirred tank reactors,distillation columns reactive columns and membrane reactors. To achievea high conversion, it is advantageous for at least one of the twoproducts, preferably all products, i.e. the carboxylic anhydride (VII)and the carboxylic acids (II) and (IIa), to be removed continually fromthe reaction system.

When using a stirred tank reactor, this is achieved, for example, bycontinuously taking off the reaction mixture, subsequently separatingoff the products and recirculating the unreacted starting materials and,if appropriate, the catalyst. The subsequent separation is generallycarried out using one or more distillation columns. It is possible todesign a separation process suitable for the specific system with theaid of customary technical skills.

The transanhydridation reaction is preferably carried out in adistillation column or reactive column. A method suitable for theprocess of the present invention is described, for example, in DE-A 3510 035. In the case of a transanhydridation-in a distillation column orreactive column, the reaction preferably occurs in-the middle region ofthe column. The carboxylic anhydride (V) and the carboxylic acid (VI)are fed in from the side in the middle part of the column. In generaland especially when using acetic anhydride as carboxylic anhydride (V),the carboxylic acids (II) and (IIa) formed, which when using aceticanhydride are identical and are both acetic acid, is the componentboiling at the lowest temperature. It is therefore generally taken offcontinuously at the top. The carboxylic anhydride (VII) formed isgenerally the component-boiling at the highest temperature and isgenerally taken off continuously at the bottom. To make an appropriatereaction zone possible in the column, it is particularly advantageous tointroduce the carboxylic anhydride (V) at a point below that at whichthe carboxylic acid (VI) is introduced, so that the reactants flow incountercurrent toward one another. Furthermore, the addition accordingto the countercurrent principle also leads to an increase in theconversion, since, for example, a high concentration of carboxylicanhydride (V) is present in the lower region of the column and, incombination with a rather low concentration of carboxylic acid (VI),shifts the equilibrium there in the direction of the desired productcarboxylic anhydride (VII). However, it is also possible to introducethe carboxylic anhydride (V) and the carboxylic acid (VI) into thecolumn together at one point. This may, for example, be advantageouswhen the two starting materials have identical or very similar boilingpoints. If a heterogeneous catalyst is used, it is preferably present inthe form of fixed packing or coatings within the column interior. Ifhomogeneous catalysts are used, these are fed into the column as furthercomponents, generally likewise continuously. Metal ions as homogeneouscatalysts are generally introduced in the upper region of the column anddischarged at the bottom, separated off from the bottom product andgenerally recirculated. Thus, for example, liquid acids which aredischarged via the bottom product are preferably introduced in the upperregion of the column. In a manner analogous to the description given forthe use of metal ions, the organic or inorganic acids discharged withthe bottoms are likewise separated off from the carboxylic anhydride(VII) and are generally recirculated. Organic or inorganic acids aregenerally introduced in the region at the opposite end of the columnfrom the point at which they are taken off, so that they are distributedover the column. Thus, for example, relatively high-boiling organic orinorganic acids which, in accordance with the conditions prevailing inthe column, are taken off at the bottom are preferably added in theupper region.

In accordance with the above discussion, it is particularly advantageousin the process of the present invention for the transanhydridation instep (c) to be carried out in a continuously operated distillationcolumn and for the reaction products carboxylic acid (II) and carboxylicanhydride (VII) formed to be taken off continuously.

Suitable apparatuses for the reaction in the gas phase are, for example,flow tubes or shaft reactors.

Any further purification of the products obtained which may be necessarycan be developed on the basis of a knowledge of the starting materials,products and, if applicable, the catalyst and using customary technicalskills.

FIG. 1 shows a block diagram of the process of the present invention.Formic ester (I) and carboxylic acid (II) are reacted in block “A”(transesterification/separation) to form formic acid (III) andcarboxylic ester (IV). The formic acid (III) separated off is dischargedas end product. The carboxylic ester (IV) separated off is passed via anoptional block “B” (discharge of carboxylic ester), in which part of thecarboxylic ester (IV) formed may be discharged as end product, to block° C.” (carbonylation). Carbon monoxide is introduced to form thecarboxylic anhydride (V). The carboxylic anhydride (V) is passed via anoptional block “D” (discharge of carboxylic anhydride), in which part ofthe carboxylic anhydride (V) formed may be discharged, as end product,to block “E” (transanhydridation). There, carboxylic acid (VI) isintroduced to form carboxylic anhydride (VII) and carboxylic acid (II)and, when an unsymmetrical carboxylic anhydride (V) is used, alsocarboxylic acid (IIa), which are discharged as products.

In a preferred embodiment of the process of the present invention, atleast part of the carboxylic acid (II) formed in step (c) is returned tostep (a). It is particularly advantageous in the process of the presentinvention for the carboxylic acid (II) to be recirculated to step (a) ina total amount approximately equal to that necessary for maintaining thecirculation there. To avoid accumulation of undesirable by-products, itmay be advantageous to recirculate slightly less carboxylic acid (II)than necessary to step (a) and to make up the difference by addition offresh carboxylic acid (II).

FIG. 2 shows a block diagram of the preferred process according to thepresent invention. The blocks “A” to “E” are as described for the blockdiagram of FIG. 1. The only difference in this preferred process is thatthe carboxylic acid (II) coming from block “E” (transanhydridation) ispassed via an optional block “F” (discharge of carboxylic acid), inwhich part of the carboxylic acid (II) formed may be discharged as endproduct, to the block “A” (transesterification/separation).

In the process of the present invention, preference is given to using aformic ester (I)

in which the radical R¹ is

-   -   an unsubstituted or substituted, unbranched or branched, acyclic        or cyclic C₁-C₁₂-alkyl radical such as methyl, ethyl, 1-propyl,        2-propyl, 1-butyl, 2-butyl, 2-methyl-1-propyl,        2-methyl-2-propyl, 1-pentyl, 2-pentyl, 3-pentyl,        3-methyl-2-butyl, 2-methyl-2-butyl, hexyl, heptyl,        2-ethyl-1-pentyl, octyl, 2,4,4-trimethyl-1-pentyl, nonyl,        1,1-dimethyl-1-heptyl, decyl, undecyl, dodecyl, phenylmethyl,        2-phenylethyl, 3-phenylpropyl, cyclopentyl, cyclopentylmethyl,        2-cyclopentylethyl, 3-cyclopentylpropyl, cyclohexyl,        cyclohexylmethyl, 2-cyclohexylethyl or 3-cyclohexylpropyl; or    -   an unsubstituted or substituted, unbranched or branched, acyclic        or cyclic C₂-C₁₂-alkenyl radical such as vinyl (ethenyl),        1-propenyl, 2-propenyl, 1-methylvinyl, 3-butenyl, cis-2-butenyl,        trans-2-butenyl, cis-1-butenyl, trans-1-butenyl, pentenyl,        hexenyl, heptenyl, octenyl, nonenyl, decenyl, 3-cyclopentenyl,        2-cyclohexenyl, 3-cyclohexenyl or 2,5-cyclohexadienyl.

Particular preference is given to using a formic ester (I) in which theradical R¹ is an unsubstituted, unbranched or branched, acyclicC₁-C₆-alkyl radical, specifically methyl, ethyl, 1-propyl, 2-propyl,1-butyl, 2-butyl, 2-methyl-1-propyl, 2-methyl-2-propyl, 1-pentyl and1-hexyl. Very particular preference is given to using methyl formate,ethyl formate, propyl formate or butyl formate, in particular methylformate.

In the process of the present invention, preference is given to using acarboxylic acid (II)

in which the radical R² is

-   -   an unsubstituted or substituted, unbranched or branched, acyclic        or cyclic C₁-C₁₂-alkyl radical, such as methyl, ethyl, 1-propyl,        2-propyl, 1-butyl, 2-butyl, 2-methyl-1-propyl,        2-methyl-2-propyl, 1-pentyl, 2-pentyl, 3-pentyl,        3-methyl-2-butyl, 2-methyl-2-butyl, hexyl, heptyl,        2-ethyl-1-pentyl, octyl, 2,4,4-trimethyl-1-pentyl, nonyl,        1,1-dimethyl-l-heptyl, decyl, undecyl, dodecyl, phenylmethyl,        2-phenylethyl, 3-phenylpropyl, cyclopentyl, cyclopentylmethyl,        2-cyclopentylethyl, 3-cyclopentylpropyl, cyclohexyl,        cyclohexylmethyl, 2-cyclohexylethyl, 3-cyclohexylpropyl,        chloromethyl, dichloromethyl, trichloromethyl; or    -   an unsubstituted or substituted, unbranched or branched, acyclic        or cyclic C₂-C₁₂-alkenyl radical such as vinyl (ethenyl),        1-propenyl, 2-propenyl, 1-methylvinyl, 3-butenyl, cis-2-butenyl,        trans-2-butenyl, cis-1-butenyl, trans-1-butenyl, pentenyl,        hexenyl, heptenyl, octenyl, nonenyl, decenyl, 3-cyclopentenyl,        2-cyclohexenyl, 3-cyclohexenyl or 2,5-cyclohexadienyl.

Particular preference is given to using a carboxylic acid (II) in whichthe radical R² is

-   -   an unsubstituted or substituted, unbranched or branched, acyclic        C₁-C₆-alkyl radical, specifically methyl, ethyl, 1-propyl,        2-propyl, 1-butyl, 2-butyl, 2-methyl-1-propyl,        2-methyl-2-propyl, 1-pentyl, 2-pentyl, 3-pentyl,        3-methyl-2-butyl, 2-methyl-2-butyl, hexyl, chloromethyl,        dichloromethyl or trichloromethyl; or    -   an unsubstituted, unbranched or branched, acyclic C₂-C₆-alkenyl        radical such as vinyl (ethenyl), 1-propenyl, 2-propenyl,        1-methylvinyl, 3-butenyl, cis-2-butenyl, trans-2-butenyl,        cis-1-butenyl, trans-1-butenyl, pentenyl or hexenyl.

Very particular preference is given to using acetic acid and propionicacid, in particular acetic acid.

In the process of the present invention, preference is given to using acarboxylic acid (VI)

in which the radical R³ is

-   -   an unsubstituted or substituted, unbranched or branched, acyclic        or cyclic C₂-C₃₀-alkyl radical such as ethyl, 1-propyl,        2-propyl, 1-butyl, 2-butyl, 2-methyl-1-propyl,        2-methyl-2-propyl, 1-pentyl, 2-pentyl, 3-pentyl,        3-methyl-2-butyl, 2-methyl-2-butyl, hexyl, heptyl,        2-ethyl-1-pentyl, octyl, 2,4,4-trimethyl-1-pentyl, nonyl,        1,1-dimethyl-1-heptyl, decyl, undecyl, dodecyl, tridecyl,        tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl,        nonadecyl, eicosyl, henicosyl, docosyl, tricosyl, tetracosyl,        pentacosyl, hexacosyl, heptacosyl octacosyl, nonacosyl,        triacontyl, phenylmethyl, 2-phenylethyl, 3-phenylpropyl,        cyclopentyl, 2-carboxycyclopentyl, cyclopentylmethyl,        2-cyclopentylethyl, 3-cyclopentylpropyl, cyclohexyl,        2-carboxycyclohexyl, cyclohexylmethyl, 2-cyclohexylethyl,        3-cyclohexylpropyl, chloromethyl, dichloromethyl or        trichloromethyl;    -   an unsubstituted or substituted, unbranched or branched, acyclic        or cyclic C₂-C₃₀-alkenyl radical, C₂-C₃₀-alkadienyl radical or        C₂-C₃₀-alkatrienyl radical, for example vinyl (ethenyl),        1-propenyl, 2-propenyl, 1-methylvinyl, 3-butenyl, cis-2-butenyl,        trans-2-butenyl, cis-1-butenyl, trans-1-butenyl, pentenyl,        hexenyl, heptenyl, octenyl, nonenyl, decenyl,        cis-8-heptadecenyl, trans-8-heptadecenyl, cis,cis-8,11        -heptadecadienyl, cis,cis,cis-8,11,14-heptadecatrienyl,        3-cyclopentenyl, 2-cyclohexenyl, 3-cyclohexenyl or        2,5-cyclohexadienyl;    -   an unsubstituted C₆-C₂₀-aryl or C₃-C₂₀-heteroaryl radical or a        C₆-C₂₀-aryl or C₃-C₂₀-heteroaryl radical substituted by one or        more C₁-C₄-alkyl radicals, for example phenyl, 2-carboxyphenyl,        2,4,5-tricarboxyphenyl, 2-methylphenyl (o-tolyl), 3-methylphenyl        (m-tolyl), 4-methylphenyl (p-tolyl), 2,6-dimethylphenyl,        2,4-dimethylphenyl, 2,4,6-trimethylphenyl, 2-methoxyphenyl,        3-methoxyphenyl, 4-methoxyphenyl, 1-naphthyl, 2-napthyl,        2-carboxy-1-napththyl, 3-carboxy-2-naphthyl,        3,6,7-tricarboxy-2-naphthyl, 8-carboxy-1-naphthyl,        4,5,8-tricarboxy-1-naphthyl, 2-carboxy-1-anthracenyl,        3-carboxy-2-anthracenyl, 3,6,7-tricarboxy-2-anthracenyl,        4,9,10-tricarboxy-3-perylenyl or        4,3′,4′,-tricarboxy-3-benzophenonyl.

The carboxylic acid (VI) used is particularly preferably propionic acid,butyric acid, pentanoic acid, hexanoic acid, 2-ethylhexanoic acid,acrylic acid, methacrylic acid, phthalic acid,benzene-1,2,4,5-tetracarboxylic acid (pyromellitic acid),benzophenone-3,3′,4,4′-tetracarboxylic acid,naphthalene-2,3,6,7-tetracarboxylic acid ornaphthalene-1,4,5,8-tetracarboxylic acid.

The carboxylic anhydride (VII) prepared in the process of the presentinvention is particularly preferably propionic anhydride, butyricanhydride, acrylic anhydride, methacrylic anhydride and/orbenzene-1,2,4,5-tetracarboxylic dianhydride (pyromellitic anhydride).

The process of the present invention is particularly preferably used toprepare

(i) formic acid (III);

(ii) acetic acid, methyl acetate and/or acetic anhydride as carboxylicacid having at least two carbon atoms (II) and/or derivatives thereof;and

(iii) propionic anhydride, butyric anhydride, acrylic anhydride,methacrylic anhydride and/or benzene-1,2,4,5-tetracarboxylic dianhydride(pyromellitic anhydride) as carboxylic anhydride (VII).

In the process of the present invention, the formic ester (I) and thecarboxylic acid (II) are generally used in the transesterification instep (a) in a ratio of 1:1, although the relative concentrations in thereaction mixture may deviate therefrom. The carboxylic acid (II) is fedin as starting material, as recycle stream from step (c) or as a mixtureof the two. Reaction of one mol of formic ester (I) and one mol ofcarboxylic acid (II) forms, in accordance with the reaction equation

one mol of formic acid (III) as product to be taken off and one mol ofcarboxylic ester (IV). Since at least part of the carboxylic ester (IV)formed in step (a) is carbonylated to the corresponding carboxylicanhydride (V), one mol of carboxylic anhydride (V) is formed from onemol of carboxylic ester (IV) in accordance with the reaction equation

Since at least part of the carboxylic anhydride (V) formed in step (b)is used in the transanhydridation with introduction of the carboxylicacid (VI), one mol of carboxylic anhydride (VII), one mol of carboxylicacid (II) and one mol of carboxylic acid (IIa) are formed from one molof carboxylic anhydride (V) and two mol of carboxylic acid (VI) inaccordance with the reaction equation

If a symmetrical carboxylic anhydride (V) is used, two mol of carboxylicacid (II) are formed since the carboxylic acids (II) and (IIa) are thenidentical. The carboxylic acid (II) formed can be taken off as productor recirculated to the transesterification in step (a).

Table 1 gives an overview of the preferred process variants andindicates the stoichiometric ratios using the formic acid (III) formedas reference parameter. The last column indicates the process blocksrequired, with the optional blocks for discharge of possibleintermediate products not being mentioned in the interests ofsimplicity.

Variant 1: Preparation of Formic Acid, Carboxylic Anhydride (VII) andAcetic Acid

A simplified process flow diagram is shown in FIG. 3. Methyl formate (I)and acetic acid (II) are fed continuously via lines (0) and (1) into thereactor (A), which is depicted by way of example as a stirred vessel.However, other suitable reaction apparatuses such as those describedabove for step (a) can also be used as reactor (A). In, the reactor (A),the transesterification to form formic acid (III) and methyl acetate(IV) takes place in the presence of the catalyst used. The reactionmixture, which comprises methyl formate (I), acetic acid (II), formicacid (III), methyl acetate (IV) and the catalyst used, is takencontinuously from the reactor (A) and passed via line (2) to the work-upby distillation, which is shown by way of example in the form of thecolumns (B), (C) and (D). Unreacted methyl formate (I) and any lowboilers formed are recirculated via line (3) to the reactor (A). Formicacid (III) is taken off via line (7). Unreacted acetic acid (II),catalyst and any high boilers formed are recirculated to the reactor (A)via line (8). It goes without saying that part of the stream (8) can, ifnecessary, be discharged continuously or discontinuously to avoidaccumulation of high boilers and, if desired, be worked up further.Methyl acetate (IV) is passed on via line (5). It is generallyadvantageous to use a dividing wall column for the two columns (B) and(C). In this case, stream (3) is taken off at the top, stream (5) istaken off as side stream and stream (6) is taken off at the bottom.

If desired, methyl acetate (IV) can be discharged via the optional line(10).

Methyl acetate (IV) is conveyed via line (9) to the carbonylation inreactor (E), which is depicted by way of example as a stirred vessel.However, other suitable reaction apparatuses, for example thosedescribed above for step (b), can also be used as reactor (E). In thereactor (E), carbonylation by means of carbon monoxide introduced vialine (11) takes place in the presence of the catalyst used to formacetic anhydride (V). The reaction mixture, which comprises unreactedmethyl acetate (IV), acetic anhydride (V) and the catalyst used, istaken continuously from the reactor (E), generally freed of thecatalyst, for example in a flash evaporator (not shown in the interestsof simplicity), and passed via line (12) to the work-up by distillation,which is shown by way of example in the form of the column (F).Unreacted methyl acetate (IV) and any low boilers formed arerecirculated to the reactor (E) via line (13). The bottom product fromthe column (F), which comprises acetic anhydride (V) and any highboilers formed, is taken off via line (14) and is generally separated ina further column (not shown in the interests of simplicity) into aceticanhydride (V) and high boilers. The catalyst-containing stream isgenerally returned to the reactor (E). It goes without saying that partof the stream comprising high boilers can, if necessary, be dischargedcontinuously or discontinuously to avoid accumulation of high boilersand, if desired, be worked up further.

If desired, acetic anhydride (V) can be discharged via the optional line(15).

Acetic anhydride (V) is conveyed continuously via line (16) to thetransanhydridation in the reactor (G), which is depicted by way ofexample as a column. In the column (G), the carboxylic acid (VI) isintroduced via line (17) and transanhydridation takes place in thepresence of the catalyst used to form the carboxylic anhydride (VII) andacetic acid (II). The product from the top of the column (G), whichcomprises acetic acid (II), unreacted acetic anhydride (V) and any lowboilers formed, is taken off via line (19) and generally fractionatedfurther in a further column (not shown in the interest of simplicity).Acetic acid (II) is taken off as product, acetic anhydride (V) isgenerally returned to the column (G) and the low boilers are discharged.As an alternative, it is naturally also possible to hydrolyze any aceticanhydride (V) present in the product from the top of the column (G) bymeans of water to form acetic acid. The product from the bottom of thecolumn (G), which comprises carboxylic anhydride (VII) and possibly thecatalyst and high boilers formed, is taken off via line (18) and isgenerally separated in a further column (not ; shown in the interest ofsimplicity) into carboxylic anhydride (V), catalyst and high boilers.The catalyst-containing stream is generally recirculated to the column(G) and the carboxylic anhydride (V) is discharged as product.

As an alternative, it is also possible to use a series arrangement of areactor, for example a stirred vessel, and one or more distillationcolumns connected in series for the work-up of the reaction mixture inplace of the column (G) for the transanhydridation.

Variant 2: Preparation of Formic Acid, Carboxylic Anhydride (VII) andAcetic Acid (With Acetic Acid Circuit)

A simplified process flow diagram is shown in FIG. 4. The acetic acid(II) fed into the reactor (A) via line (20) comes predominantly,preferably entirely, from the acetic acid circuit. However, addition ofadditional acetic acid via line (1) is also possible if necessary. Thetransesterification, the carbonylation and the transanhydridation arecarried out as described in variant 1, which is explicitly incorporatedby reference at this point.

Instead of discharging all of the acetic acid (II) formed in thetransanhydridation as product via line (19), in this preferredembodiment the acetic acid (II) necessary for the transesterification instep (a) is conveyed via line (20) back to the reactor (A), thus closingthe circuit. Excess acetic acid (II) can naturally be taken off asproduct via line (19).

Variant 3: Preparation of Formic Acid, Carboxylic Anhydride (VII) andAcetic Anhydride (With Acetic Acid Circuit)

The likewise preferred variant 3 corresponds essentially to variant 2,except that part of the acetic anhydride (V) formed is discharged asproduct via line (15) and only the proportion necessary to maintain theacetic acid circuit is passed on to the transanhydridation. Thus, all ofthe acetic acid formed in the transanhydridation is recirculated vialine (20) to the transesterification in this variant.

The process of the present invention makes it possible to prepare (i)formic acid, (ii) a carboxylic acid having at least two carbon atomsand/or derivatives thereof, for example a carboxylic ester or acarboxylic anhydride, and (iii) a further carboxylic anhydride on thebasis of readily available and economically attractive raw materials.Thus, for example, the particularly-preferred products formic acid,methyl acetate, acetic anhydride and acetic acid are based entirely onsynthesis gas and thus on natural gas as raw material.

Furthermore, the process of the present invention makes possible asimple and inexpensive construction of the plant (low capital costs), alow energy consumption and low operating costs. Due to the coupling ofthe preparation of formic acid and a carboxylic acid having at least twocarbon atoms and/or derivatives thereof, a plant operating according tothe process of the present invention requires a significantly lowercapital investment than two separate plants according to the prior art.In particular, the two separate plants according to the prior art aredispensed with. Furthermore, the circuit route via toxic ketene whichrequires a great deal of energy for its preparation is dispensed with inthe preparation of acetic anhydride by the process of the presentinvention.

The process of the present invention avoids the formation of undesirableby-products as a result of coupled production.

Furthermore, the process of the present invention also makes itpossible, if required, to prepare anhydrous formic acid and anhydrouscarboxylic acids which are significantly less corrosive than thewater-containing compounds and thus offer increased safety and allow theuse of cheaper materials of construction. As a result of the simple(compared to the prior art) and economically attractive route tovirtually anhydrous formic acid, a particularly high formic acid qualityis achieved. The very low residual water content also results in anadvantage in transport and storage of the formic acid prepared in thisway.

Furthermore, the process of the present invention offers a high degreeof flexibility in terms of the carboxylic acid having at least twocarbon atoms and/or derivatives thereof, since the relative amounts ofthe compounds discharged can be varied within a wide range according torequirements. Additional introduction of an alcohol into thecarbonylation step enables the ratio of the carbonylation products toformic acid to be increased. Thus, there is also a high degree offlexibility in respect of increased production of carbonylation productsand their downstream products.

In the preferred preparation of acetic acid and its derivatives, theprocess of the present invention offers the further advantage of beingable to carry out the carbonylation of methyl acetate in the absence ofwater and thus achieve a higher yield from the carbon monoxide usedcompared to the industrially customary carbonylation of methanol byavoiding the water gas shift reaction.

As a result of the use of the acetic anhydride prepared in aparticularly advantageous manner as anhydride formation reagent forcarboxylic acids, in particular propionic acid, butyric acid, acrylicacid and methacrylic acid, and the recirculation of the acetic acidformed to the acetic acid circuit, the preparation of a wide variety ofcarboxylic anhydrides from the parent carboxylic acids is alsoparticularly advantageous. TABLE 1 Preferred embodiments with indicationof the idealized stoichiometric ratios Starting materials ProductsProcess blocks 1 (I): Methyl formate (III): Formic acid A, C, E (II):Acetic acid (II): 2 acetic acid Carbon monoxide (VII): Carboxylicanhydride (VI): 2 carboxylic acid 2 (I): Methyl formate (III): Formicacid A, C, E, F Carbon monoxide (II): Acetic acid Acetic acid (II) incircuit (VI): 2 carboxylic acid (VII): Carboxylic anhydride 3 (I):Methyl formate (III): Formic acid A, C, D, E Carbon monoxide (II): ½Acetic anhydride Acetic acid (II) in circuit (VI): Carboxylic acid(VII): ½ Carboxylic anhydride

1. A process for the joint preparation of (i) formic acid (III); (ii) acarboxylic acid having at least two carbon atoms (II) and/or derivativesthereof; and (iii) a carboxylic anhydride (VII); said processcomprising: (a) transesterifying a formic ester (I) with a carboxylicacid having at least two carbon atoms (II) to form formic acid (III) andthe corresponding carboxylic ester (IV); (b) carbonylating at least partof the carboxylic ester (IV) formed in step (a) to form thecorresponding carboxylic anhydride (V); and (c) transanhydriding atleast part of the carboxylic anhydride (V) formed in step (b) with acarboxylic acid (VI) to form a carboxylic anhydride (VII) and thecarboxylic acid (II).
 2. The process according to claim 1, wherein (d)at least part of the carboxylic acid (II) formed in step (c) isrecirculated to step (a).
 3. The process according to claim 1, whereinthe transanhydridation in step (c) is carried out in the presence of anacidic or basic ion exchanger or an acidic or basic oxide.
 4. Theprocess according to claim 1, wherein the transanhydridation in step (c)is carried out in the presence of an organic or inorganic acid which hasa pK_(a) which is lower than that of the carboxylic acid (VI) and thecarboxylic acid (II).
 5. The process according to claim 1, wherein thetransanhydridation in step (c) is carried out in the presence of a metalion from groups 1 to 13 of the Periodic Table.
 6. The process accordingto claim 1, wherein the transanhydridation in step (c) is carried out ina continuously operated distillation column and the reaction productscarboxylic acid (II) and carboxylic anhydride (VII) formed arecontinuously taken off.
 7. The process according to claim 1, wherein theformic ester (I) used is methyl formate.
 8. The process according toclaim 1, wherein the carboxylic acid (II) used is acetic acid.
 9. Theprocess according to claim 1, wherein the carboxylic anhydride (VII)prepared is at least one carboxylic anhydride (VII) selected from thegroup consisting of propionic anhydride, butyric anhydride, acrylicanhydride, methacrylic anhydride and benzene-1,2,4,5-tetracarboxylicdianhydride.
 10. The process according to claim 1, wherein (i) formicacid (III) is prepared; (ii) the carboxylic acid having at least twocarbon atoms (II) derivatives thereof prepared is at least onecarboxylic acid selected from the group consisting of acetic acid,methyl acetate and acetic anhydride; and (iii) the carboxylic anhydride(VII) prepared is at least one carboxylic anhydride (VII) selected fromthe group consisting of propionic anhydride, butyric anhydride, acrylicanhydride, methacrylic anhydride and benzene-1,2,4,5-tetracarboxylicdianhydride.
 11. The process according to claim 2, wherein thetransanhydridation in step (c) is carried out in the presence of anacidic or basic ion exchanger or an acidic or basic oxide.
 12. Theprocess according to claim 2, wherein the transanhydridation in step (c)is carried out in the presence of an organic or inorganic acid which hasa pKa which is lower than that of the carboxylic acid (VI) and thecarboxylic acid (II).
 13. The process according to claim 2, wherein thetransanhydridation in step (c) is carried out in the presence of a metalion from groups 1 to 13 of the Periodic Table.
 14. The process accordingto claim 2, wherein the transanhydridation in step (c) is carried out ina continuously operated distillation column and the reaction productscarboxylic acid (II) and carboxylic anhydride (VII) formed arecontinuously taken off.
 15. The process according to claim 3, whereinthe transanhydridation in step (c) is carried out in a continuouslyoperated distillation column and the reaction products carboxylic acid(II) and carboxylic anhydride (VII) formed are continuously taken off.16. The process according to claim 4, wherein the transanhydridation instep (c) is carried out in a continuously operated distillation columnand the reaction products carboxylic acid (II) and carboxylic anhydride(VII) formed are continuously taken off.
 17. The process according toclaim 5, wherein the transanhydridation in step (c) is carried out in acontinuously operated distillation column and the reaction productscarboxylic acid (II) and carboxylic anhydride (VII) formed arecontinuously taken off.
 18. The process according to claim 2, whereinthe carboxylic acid (II) used is acetic acid.
 19. The process accordingto claim 3, wherein the carboxylic acid (II) used is acetic acid. 20.The process according to claim 4, wherein the carboxylic acid (II) usedis acetic acid.